Life cycle and host range of Phycita sp. rejected for biological control of prickly acacia in Australia

Abstract

<p>Prickly acacia (<i>Vachellia nilotica</i> subsp. <i>indica</i>), a native of the Indian sub-continent, is a serious weed of the grazing areas of northern Australia and is a target for classical biological control. Native range surveys in India identified a leaf webber, <i>Phycita</i> sp. (Lepidoptera: Pyralidae) as a prospective biological control agent for prickly acacia. In this study, we report the life cycle and host-specificity test results <i>Phycita</i> sp. and highlight the contradictory results between the no-choice tests in India and Australia and the field host range in India. In no-choice tests in India and Australia, <i>Phycita</i> sp. completed development on two of 11 and 16 of 27 non-target test plant species, respectively. Although <i>Phycita</i> sp. fed and completed development on two non-target test plant species (<i>Vachellia planifrons</i> and <i>V. leucophloea</i>) in no-choice tests in India, there was no evidence of the insect on the two non-target test plant species in the field. Our contention is that oviposition behaviour could be the key mechanism in host selection of <i>Phycita</i> sp., resulting in its incidence only on prickly acacia in India. This is supported by paired oviposition choice tests involving three test plant species (<i>Acacia baileyana</i>, <i>A. mearnsii</i> and <i>A. deanei</i>) in quarantine in Australia, where eggs were laid only on prickly acacia. However, in paired oviposition choice trials, only few eggs were laid, making the results unreliable. Although oviposition choice tests suggest that prickly acacia is the most preferred and natural host, difficulties in conducting choice oviposition tests with fully grown trees under quarantine conditions in Australia and the logistic difficulties of conducting open-field tests with fully grown native Australian plants in India have led to rejection of Phycita sp. as a potential biological control agent for prickly acacia in Australia.</p>